Раздел III. Тексты для внеаудиторного чтения

Литература:

Маркевич Т.А. Практикум “Diesel Engines” к практическим занятиям и самостоятельной работе для курсантов 4 курса специальности 26.05.06 «Эксплуатация судовых энергетических установок» очной и заочной формы обучения. Керчь, 2016 г.

1. DIESEL ENGINE: PRINCIPLE OF OPERATION (c.5-6)

2. FUEL SYSTEMS (c.65-71)

3. THE COOLING SYSTEM (c.76-79)

4. THE LUBRICATING OIL SYSTEM (c.85-88)

3.1 DIESEL ENGINE: PRINCIPLE OF OPERATION

A device which burns fuel creating heat to perform work is a heat engine. Heat engines can be classified as external combustion, such as steam boiler, or internal combustion (IC). They can further be divided into the spark ignited (SI) engine or the compression ignited engine (CI). The Compression Ignited heat engine has been most famously referred to as the Diesel engine, named after its inventor, Rudolph Diesel.

Diesel engines are furthermore sub-divided into three categories: slow, medium and high speed. Slow speed are considered to be up to 300 rpm such as most big two stroke engines commonly found on ships. Medium speed engines dwell in the 300 - 900 rpm range. They are most common on smaller ships and power plants driving electrical generators and / or the propeller. High speed engines are the most common.

The diesel engine is a type of internal combustion engine, which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines, the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or in two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.

The fundamental requirements for the operation of a diesel engine are a supply of fuel, the necessary air for combustion of the fuel and some means to get the air and fuel into the cylinders and the products of combustion out.

The stages in the operation of a diesel engine are as follows:

1. Supply of air.

2. Compression of the air to raise its temperature high enough to initiate combustion of the fuel.

3. Supply of fuel.

4. Expansion of the hot high-pressure gas, which forces out the piston against the resistance of the load on the crankshaft.

5. Removal of the products of combustion.

These stages may be performed in two or four strokes of the piston (one or two revolutions of the engine crank).

A diesel engine produces power only when it is burning fuel. It is possible both to calculate the heat content in Btus of the fuel burned and to figure the Btu equivalent of the horsepower produced (l h.p. = 2,544 Btus). In a perfect engine all the heat from the burning fuel would be converted into useful energy; as the piston descended on the power stroke, the pressure and temperature in the cylinder would decrease to exactly the same values that existed at the beginning of the cycle.

In practice, considerable heat and pressure remain at the end of the power stroke, and must be removed to enable a fresh charge of air to be drawn in and to prevent the build-up of dangerously high temperatures that would damage the engine. The net result is that the average diesel engine converts into usable energy just 30 to 40% of the heat generated. The rest is dissipated as cooling water, 25%-30%; exhaust gases, 25%-30%; and internal friction, radiation from the engine block, and related losses, 10%. As bad as this sounds, it is still considerably more efficient than a gasoline engine.

The diesel engine has no ignition system. The injected fuel is ignited by the temperature rise associated with compressing air to a high pressure. The ignition point of diesel fuel is about 750°F, but in practice, most diesel engines compress the air until a temperature of about 1,000°F is achieved.

3.2 FUEL SYSTEMS (c.65-71)

The fuel oil system for a diesel engine can be considered in two parts-the fuel supply and the fuel injection systems. Fuel supply deals with the provision of fuel oil suitable for use by the injection system.

The equipment used on board a diesel-driven ship for han­dling and controlling fuel oil belongs to several separate systems.

Fuel injection system performs the following functions: (1) it meters the quan­tity of the fuel required by the engine and maintains this quantity constant; (2) it injects fuel at the correct point in the cycle at all engine speeds and loads; (3) it begins and ends injection very quickly; (4) it injects fuel at the rate necessary to control com­bustion; (5) it atomizes fuel in the combustion chamber of each cylinder. The most important devices of this system are high-pressure fuel pumps and fuel injection valves.

The function of the fuel injection system is to provide the right amount of fuel at the right moment and a suitable condition for the combustion process. There must therefore be some form of metered supply, a means of timing the delivery, of atomisation and distribution of fuel.

There are two basic systems in use. One is the common rail system, in which a single pump supplies fuel at high pressure to a common manifold feeding the cylinders.

Injection of the fuel to each cylinder takes place through a fuel valve operated from the camshaft which releases a metered amount of fuel into each cylinder at the instant it is required.

The principal build of a common rail fuel system for diesel engines.

The fuel is drawn up by a high pressure plunger pump and pumped into a spacious collection pipe; this pipe is known as the ‘common rail’. The fuel pressure in this pipe is kept constant. A control-unit regulates the fuel injection with the injector.

The greater the fuel demand, the greater the fuel supply. This is measured using a quantity measurement on the suction line of the pumps.

The other system is known as the jerk pump system, in which the fuel is metered and raised in pressure by a separate fuel pump for each cylinder. The pump is timed to force the fuel through the injector into the cylinder at the appropriate moment. In this system the high-pressure fuel pump and the injector have been placed in one casing. The fuel injector is situated in the cylinder and is mechanically driven from the cam shaft, as are the inlet- and exhaust valves, by means to a fuel cam, guide pulley, push rod and lever.

The great majority of medium and slow speed engines use the latter system.

The plunger is actuated by a cam and a roller follower. A helical spring is fitted to return the plunger on its down stroke and to maintain contact of follower on the cam.

When the follower is on the base circle of the cam, the pump plunger is at the bottom of its stroke and the inlet port in the barrel is uncovered allowing the fuel to fill that portion of the barrel above the plunger.

The plunger is a close fit within a barrel. As the cam rotates the plunger rises and seals off the inlet and relief ports and at this point of the stroke the pumping action starts.

Further upward movement of the plunger causes the fuel to be raised in pressure and expelled through the delivery valve to the injector. A helical groove (or helix) extends from the top of the plunger part way down its cylindrical surface.

When the edge of the helix uncoveres the relief port, the high pressure in the fuel above the plunger is released and pumping ceases / See Fig.11.2.(b)/, altough the plunger continues to move upwards.

The amount of fuel delivered will vary in accordance with the effective length of the stroke. This is controlled by rotating the plunger in the barrel by means of rack and pinion, the latter being machined on the outside of a sleeve. The sleeve fits over the plunger engaging it with keys.The rack position, therefore, determines the quatity of fuel supplied.

The timing of the injection is controlled by the instant that the pump plunger closes the inlet and relief ports. This instant can be adjusted with the reference to the camshaft and crankshaft position by raising or lowering the plunger by the screw in the tappet. Raising the level of the screw will advance the point of injection.

After leaving the pump delivery valve, the fuel is conveyed by high pressure steel piping to the injector. The fuel flows at high velocity through small holes in the injector nozzle causing it to divide up into fine spray which penetrates throughout the combustion chamber.

The high pressure of the fuel necessary to do this must be created sharply at the commencement of injection and must be just as sharply dropped when the injection ceases in order to avoid dribbling.

VOCABULARY

value to handle wax pipe blocking remains intermittent pressure pressure maintenance valve shut off valve non return valve to prevent leakage overflow line surplus fuel finely atomized orifice booster pump fuel injection valve fuel handling purification clarification bowel sludge fuel piping system fuel heating pipes fuel nozzle leakage fuel pump capacity counter-pressure pressure-retaining valve double cone seat to prevent leakage to be isolated single-controlled pump to deliver fuel discharge line camshaft cam roller pushrod rocking lever high-pressure fuel pump fuel injection valve needle nozzle loaded spring overflow pressure retaining valve

величина перекачивать парафин засорение трубы остатки пульсирующее давление клапан постоянного давления отсечной клапан невозвратный клапан предотвращать утечку трубопровод перелива избыточное топливо мелко распыленный отверстие подкачивающий насос топливо-подкачивающий насос обработка топлива очистка топлива (от примесей) очистка топлива (осветление) барабан отстой, грязь трубопровод топливной системы трубопровод подогрева топлива топливное сопло спуск топлива производительность насоса противодавление клапан постоянного давления двуконусное седло предотвращать утечку быть отключенным насос основной подавать топливо трубопровод подачи| кулачковый вал кулачок ролик толкатель качающийся рычаг насос высокого давления топливо-впрыскивающий клапан игла сопло нагрузочная пружина слив клапан, сохраняющий определенное заданное давление

3.3 THE COOLING SYSTEM (c.76-79)

The purpose of a cooling system is to maintain a constant temperature throughout the engine that minimises hot spot expansion leading to seizing of the moving parts.

The cooling system of a marine diesel engine functions to keep the engine parts and fluids at safe operating temperatures. In the open system, the engine is cooled directly by seawater. In the closed system, fresh water is circulated through the engine. The fresh (jacket) water is then cooled as it passes through a cooling device where heat is carried away by a constant flow of seawater or air. The closed cooling system is the design that is most commonly used on marine internal combustion engines.

THE OPEN COOLING SYSTEM

An open system means that the liquid that is used to carry heat away from the engine is drawn directly from the water in which the boat or ship operates. This liquid is moved through the system and then discharged overboard. In the open system, there is no freshwater circuit.

The open cooling system is not used on most marine diesel engines for several reasons. The most important reason is that the open cooling system exposes the engine to scale formation, marine growth, and dirt deposits in the piping. You may, however, find the open cooling system in use on some small gasoline engines, such as outboard motors and P-250 fire-fighting pumps.

THE CLOSED COOLING SYSTEM

A cooling system is classified as closed if it has a freshwater circuit (system) that is selfcontained and used continuously for the cooling of the engine. Closed cooling systems are normally operated at pressures greater than atmospheric pressure so that the boiling point of the coolant is raised to a temperature that is higher than 212°F.

Cooling of an internal-combustion engine is accomplished by the use of either a cooler (heat exchanger), keel cooler, or radiator and fan.

The heat exchanger cooling system combines two separate cooling systems—a jacket-water (freshwater) system and the raw-water (seawater) cooling system. The principal components that comprise the freshwater system are an engine coolant pump, one side of the heat exchanger, and the expansion tank and piping.

Heat exchanger cooling

The heat exchanger cooling system maintains a constant temperature throughout the engine by removing heat from the hottest part of the engine in the vicinity of the combustion space and transfers it to the cooler parts. Used salt cooling water is ejected directly overboard or via the exhaust. Marine systems are designed to operate at temperatures of typically 85°- 90° C and may be pressurised to raise coolant boil temperature and provide more efficient circulation. An engine can operate intermittently up to 96° with a header tank cap pressure of 103 kPa (1.03 bar or 15 psi). The cooling water high temperature alarms are set at around 96° C.

In the schematic system shown below, engine driven pump sucks sea water from the sea chest through the cooler (header tank and heat exchanger) and another circulates fresh water through the engine water jackets and the cooler. The close contact of the engine’s hot fresh water is cooled (heat exchanged) by the piped cold salt water on its way to be ejected (and cool) the exhaust. The water cooling system for a slow speed diesel engine consists of two separate circuits: one for cooling the cylinder jackets, cylinder heads and turboblowers; the other for piston cooling. A separate piston cooling system is used to prevent any possibility of contamination from piston cooling glands.

The jacket cooling system is a closed circuit. Water passing from the engine returns through a cooler to the circulating pump and then to the engine. A header or expansion tank is placed at a sufficient height to allow the venting and water make-up in the system. This has connection from the engine discharge and to the pump suction line. A heater is included with by-pass to warm the engine prior to starting by circulating hot water.

Water enters at the lower end of the jackets, passing up to the cylinder covers and then to the exhaust valve cages, if these are fitted. Some water is taken from the discharge and passed through the turbo-charger turbine cooling spaces, before returning to the main discharge.

The piston cooling system pump draws from the supply (or drain) tank passing water to the piston cooler and then to the engine piston distribution manifold. The return from these flows by gravity to the supply tank. Arrangements may also be included for the return of any leakage from the glands. This must first pass through an oil separator and inspection tank. A steam coil is fitted in the piston cooling water supply tank for preparing the engine for sea.

All fresh water coolers are circulated with the salt (or raw) water and have by-pass valve fitted. Thermostatic valves are provided to regulate the flow of either the fresh water or sea water and so control the temperature of water passing through the engine. Fresh water pressure should always be greater than that of the salt water to prevent any possibility of salt water entering the engine system. To reduce the corrosive action and inhibit the formation of scale deposit in the system it is usual to provide some form of water treatment.

Both jacket and piston cooling systems must have alarms fitted to give warning of loss in pressure, high or low tank level or, in some cases, excess of temperature. On most engines the fresh water and sea water pumps are both of the centrifugal type. They may be engine driven or they may be separately driven by electric motors.

VOCABULARY

cooling system main engine cooling system cooling capacity exhaust manifold exhaust manifold jacket crosshead guides cooling medium independent circuit cylinder jacket cage exhaust valve cage jacket system storage tank to prevent leakage enclosed fresh water system salt-water method troubles due to scale, sediment circulating pump sea water pump drain cock plug frictional heat rubbing surface to remove overheating unequal to transfer cooler amount to absorb to vary water-to-water water-to-air efficiency circulating heat-exchanger jacket brake horsepower

система охлаждения система охлаждения главного двигателя эффективность охлаждения выхлопной коллектор зарубашечное пространство выхлопного коллектора направляющие крейцкопфа охлаждающая среда отдельный контур рубашка цилиндра корпус корпус выхлопного клапана система зарубашечного пространства отстойный танк предотвращать утечку замкнутая система пресной воды метод охлаждения морской водой неполадки из-за накипи, отложений циркуляционный насос насос забортной воды спускной клапан заглушка тепло при трении соприкасающаяся поверхность удалить перегрев неравномерный передавать охладитель, холодильник количество поглощать изменять(ся) водяной воздушно-водяной эффективность, производительность, к.п.д. циркулирующий теплообменник рубашка, зарубашечное пространство эффективная мощность

3.4 THE LUBRICATING OIL SYSTEM (c.85-88)

Proper lubrication is critical to successful engine operation. The lubrication system of a modern engine accomplishes three primary purposes:

• It lubricates surfaces to minimize friction losses.

• It cools internal engine parts that cannot be directly cooled by the engine’s water-cooling system.

• It cleans the engine by flushing away wear particles.

Additionally, the lubricant itself performs other functions:

• It cushions the engine’s bearings from the shocks of cylinder firing.

• It neutralizes the corrosive elements created during combustion.

• It seals the engine’s metal surfaces from rust.

The oil system provides a constant supply of filtered oil to the engine. Main bearings, piston cooling jets, camshafts, gear train, rocker arms, and turbocharger bearings are just a few of the components that require proper lubrication for normal function.

The oil is taken from the drain tank usually underneath the engine by a screw type pump. It is cooled, filtered and supplied to the engine via the oil inlet pipe or inlet rail at a pressure of about 4 bar. On a medium speed 4 stroke engine the oil is supplied to the main bearings through drillings in the engine frame to the crankshaft main bearings. Drillings in the crankshaft then take the oil to the crankpin or bottom end bearings.

The oil is then led up the connecting rod to the piston or gudgeon pin and from there to the piston cooling before returning to the crankcase.

Oil is also supplied to lubricate the rocker gear operating the inlet and exhaust valves, and to the camshaft and camshaft drive.

The oil then drains from the crankcase into the drain tank or sump. The oil in the drain tank is being constantly circulated through a centrifugal purifier. This is to remove any water and products of combustion plus any foreign particles which may be in the oil.

The cylinder liner must be lubricated as well. This is so there will be a film of oil between the piston rings and the liner and also so that any acid produced by combustion of the fuel is neutralised by the oil and does not cause corrosion. Some of this lubrication will be supplied by so called "splash lubrication" which is the oil splashed up into the liner by the rotating crankshaft. However larger medium speed marine diesel engines also use separate pumps to supply oil under pressure to the cylinder liner. The oil is led through drillings onto the liner surface where grooves distribute it circumferentially around the liner, and the piston rings spread it up and down the surface of the liner.

A pre lub pump is sometimes fitted especially to engines where the main pump is engine driven. This pump is electrically driven and circulates oil around the engine prior to starting.

On a two stroke crosshead engine lubricating oil is supplied to the main bearings and camshaft and camshaft drive. A separate supply is led via a swinging arm or a telescopic pipe to the crosshead where some of it is diverted to cool the piston (travelling up and back through the piston rod), whilst some is used to lubricate the crosshead and guides, and the rest led down a drilling in the connecting rod to the bottom end or crankpin bearing. Oil is also used to operate the hydraulic exhaust valves.

On some engines, the oil supply to the crosshead bearing is boosted in pressure to about 12 bar by a second set of pumps. This oil is also used to operate the hydraulic reversing gear for the engine.

The cylinder liners on a two stroke engine are lubricated using separate injection pumps which use a different specification of oil. The oil which is led to drillings in the liner is able to deal with the acids produced by the burning of high sulphur fuels.

VOCABULARY

seal to oxidize to squeeze (out) to stick to assemble to draw to discharge cooler manifold hole crankpin wristpin interior detergent oils dry-sump system wet-sump engine dipstick relief valve connecting rod camshaft governor cam follower grease to splash valve gear circumferential groove to prevent air lock to suit spring plug gauge to remove strainer alkali Total Base Number demulsibility sludge corrosion inhibition carbon deposits additives detergent additives anticorrosive additives alloy additives antiwear additives coarse filtering fine filtering solid particles contaminated oil contaminant ferrous particles servicing indicator vent plug flow out to in body drain differential pressure connections

герметизация окислять выжимать, выдавливать застревать, заедать, залипать, слипаться собирать засасывать подавать холодильник коллектор отверстие мотылевый шатунный внутренний моющие масла сухой картер мокрый картер мерная рейка предохранительный клапан шатун кулачковый вал регулятор ролик толкателя консистентная смазка разбрызгивать механизм привода клапана круглый паз предотвратить образование воздушной пробки соответствовать пружина заглушка манометр снимать сетчатый фильтр щелочь щелочное число деэмульгирующая способность шлам замедление коррозии частицы нагара присадки моющие присадки (к маслу ) антикоррозийные присадки легирующие присадки противоизносные присадки грубая очистка тонкая очистка твердые частицы загрязненное масло загрязняющее вещество железные частицы индикатор работы фильтра пробка выпуска воздуха направление движения неочищенного масла сливная npoбка места присоединения манометров